1. Field of the Invention
The present invention relates to a method and apparatus of electrophotography utilized for a printer or a copying machine and, more particularly, to a method and apparatus for forming a toner image utilizing a photoreceptor having a photoconductive layer and a toner having no photoconductivity.
2. Discussion of the Background
Electrophotography generally refers to an image-forming technique combining the photoconductive effect and the electrostatic attraction phenomenon. In each method applied to a copying machine or the like among the image forming techniques utilizing electrophotography, a developmental step is required in which an electrostatic latent image is formed on a photoreceptor and is converted into a toner image. This has prevented production of a more compact and inexpensive image-forming apparatus. Furthermore, the development step of an electrostatic latent image leads to a degradation in the picture quality due to the edge effect in which the field strength differs between the central and peripheral portions of the electrostatic latent image.
In order to solve this problem, various attempts have been made as in U.S. Pat. No. 2,924,519, Japanese Patent Disclosure (Koukoku) No. 38-22645, and Japanese Patent Disclosure (Koukai) No. 49-76531. These methods form a toner image in accordance with the following processes. First, a charged photoconductive toner is uniformly applied on a grounded electrically conductive support. The toner layer is exposed in accordance with the image density of the original object (copy) to selectively weaken the electrostatic attractive force acting between the support and toner. The toner in the exposed region with the weakened electrostaic attractive force is transferred to toner-receiving paper. Alternatively, after the toner in such a region is removed, the residual toner is transferred onto the same paper. In this manner, the toner image is formed on the toner-receiving paper.
However, with such a method, the effective sensitivity of the photoconductive toner is considerably lower than that of a photoreceptor used in other electrophotography techniques. This may be attributed to the following. First, during exposure, the light does not reach in a sufficient amount the deep region of the toner layer (i.e., the region near the support of the toner layer). Second, since the contact resistance between the toner particles is great, the charge generated upon exposure has difficulty reaching the support. If the sensitivity of the photoconductive toner is low, the density of the toner image is lowered, and fog occurs around the toner image, thus degrading picture quality. A photoconductive toner having a high sensitivity has not been proposed.